Automatic slope adjustment for bi-levelpressure support system

ABSTRACT

An apparatus and method for a bi-level positive airway pressure support in which the rise time from the expiratory positive airway pressure to the inspiratory positive airway pressure is automatically controlled by the pressure support system. The pressure support system includes a sensor, a control system, and a pressure generating system. The sensor monitors the patient&#39;s respiration to detect respiratory events, such as an apnea, hyponea or other disturbance, and the control system responds to the sensor information to adjust the rise time from the expiratory positive airway pressure to the inspiratory positive airway pressure gas pressure to maximize patient comfort and pressure support treatment effectiveness.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims priority under 35 U.S.C. §120 as aContinuation of U.S. patent application Ser. No. 09/896,644 filed Jun.29, 2001, which claims priority under 35 U.S.C. §119(e) from U.S.provisional patent application No. 60/216,999 filed Jul. 10, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a bi-level positive airwaypressure support system, and, more particularly, to a bi-level pressuresupport system and method of providing bi-level pressure support inwhich the slope of a transition of pressure from the expiratory phase ofpressure support to the inspiratory phase is automatically adjusted.

2. Description of the Related Art

Pressure support systems that provide a flow of breathing to an airwayof a patient at an elevated pressure to treat a medical disorder arewell known. One basic form of pressure support system is a continuouspositive airway pressure (CPAP) system, which typically involvesproviding a flow of breathing gas, such as air, to a patient's airway ata constant pressure throughout a patient's breathing cycle. When used totreat obstructive sleep apnea (OSA), for example, this constant pressureis provided at a level sufficient to overcome a patient's airwayresistances.

It is also known to provide a bi-level positive pressure therapy inwhich the pressure of gas delivered to the patient varies with thepatient's breathing cycle. In a bi-level pressure support system, aninspiratory positive airway pressure (IPAP) is provided during apatient's inspiratory phase of the breathing cycle and an expiratorypositive airway pressure (EPAP) is provided during the expiratory phase.The EPAP is lower than the IPAP so that the patient exhales againstrelatively low pressure as compared to the IPAP pressure, therebyincreasing the comfort to the patient. The BiPAP® family of pressuresupport devices manufactured by Respironics, Inc. of Murrysville, Pa.,are examples of pressure support device that provide this bi-level formof pressure support therapy. In addition, several U.S. patents describethis bi-level pressure support system in detail, including U.S. Pat.Nos. 5,433,193; 5,313,937; 5,239,995; and 5,148,802, all of which arehereby expressly incorporated herein by reference as if set forth intheir entirety herein.

With the improved effectiveness of bi-level pressure support systemsover their progeny, CPAP systems, the emphasis has shifted to creatingbi-level pressure support systems that are more comfortable for apatient to use without sacrificing treatment effectiveness. It isanticipated that a more comfortable pressure support system will be morefrequently and more correctly used by the patient.

U.S. Pat. No. 5,927,274 discloses a bi-level pressure support systemthat transitions from EPAP and IPAP over a rise time interval, whichtypically has a length of several hundreds of milliseconds. The '274patent provides the operator with the ability to manually adjust thisrise time interval to increase patient comfort. While the manual risetime selection technique taught by the '274 patent is a step towardincreasing patient comfort, it is also burdensome, because the operatormust manually adjust the rise time setting as needed via a control inputon the pressure support device. This can require numerous adjustmentsover relatively short periods of time if patient comfort is to beoptimized. It can be appreciated that there is perceived a need for apressure support system with increased patient comfort with little or noresultant decrease in therapy effectiveness and that minimizes theamount of operator intervention required to implement the improvedpressure support therapy effectively.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide abi-level pressure support system that overcomes the shortcomings ofconventional pressure support systems. This object is achieved accordingto one embodiment of the present invention by providing a bi-levelpressure support system that includes a pressure generating system thatproduces a flow of breathing gas at an inspiratory positive airwaypressure and an expiratory positive airway pressure. A conduit deliversthe flow of breathing gas to an airway of a patient. A sensor detects aphysiological condition of the patient, such as whether the patient isexperiencing a breathing disorder. A control system controls the outputof the pressure generating system to automatically adjust the slope of atransition of pressure from the inspiratory positive airway pressure tothe expiratory positive airway pressure based on the output of thesensor. Preferably, the control system increases the slope in theabsence of breathing disorders to increase patient comfort.

It is yet another object of the present invention to provide a method ofproviding bi-level pressure support that does not suffer from thedisadvantages associated with conventional pressure support techniques.This object is achieved by providing a method that includes producing aflow of breathing gas at an inspiratory positive airway pressure and anexpiratory positive airway pressure that is less than the inspiratorypositive airway pressure, detecting a physiological condition of apatient receiving the flow of breathing gas, and determining a slope ofthe pressure transition from the inspiratory positive airway pressure tothe expiratory positive airway pressure based on the physiologicalcondition of the patient. The rate of change from the inspiratorypositive airway pressure to the expiratory positive airway pressure iscontrolled based on this slope.

These and other objects, features and characteristics of the presentinvention, as well as the methods of operation and functions of therelated elements of structure and the combination of parts and economiesof manufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious figures. It is to be expressly understood, however, that thedrawings are for the purpose of illustration and description only andare not intended as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a theoretical pressure curve, and FIGS. 1B, 1C, and 1Dshow a pressure curve having an average rise time (FIG. 1B), a pressurecurve having a shorter rise time (FIG. 1C), and a pressure curve havinga longer rise time (FIG. 1D) that are capable of being generated by thepressure support system of the present invention;

FIG. 2 is a schematic block diagram of a positive airway pressuresupport system with automatically adjustable rise time according to onepreferred embodiment of the present invention; and

FIG. 3 is a flow chart of the control system used in one preferredembodiment of the pressure support system of the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS OF THEINVENTION

As discussed above, a bi-level pressure support system provides aninspiratory positive airway pressure (IPAP) during inhalation and anexpiratory positive airway pressure (EPAP) during exhalation to theairway of a patient. For most patients requiring bi-level therapy, ahigher IPAP pressure is required to maintain airway patency duringinhalation, and a much lower EPAP pressure is sufficient to maintainairway patency during exhalation. In fact, it is known to set the EPAPlevel as low as atmospheric pressure for some patients. By providingbi-level pressure support with the lowest necessary EPAP pressure, thework required for the patient to exhale is reduced and, therefore, thepatient's comfort is increased. This, in turn, promotes patientcompliance with the prescribed therapy.

FIG. 1A schematically depicts a theoretical pressure curve 100 output bya bi-level positive airway pressure support system over a portion of apatient's breathing cycle. During an expiratory phase of the breathingcycle, pressure curve 100 is at the expiratory pressure 105. At the endof exhalation, i.e., at the onset of the subsequent inhalation, pressurecurve 100 changes to an inspiratory pressure 115. When the systemdetects the end of inspiration, i.e., at the onset of the subsequentexhalation, pressure curve 100 returns to the lower expiratory pressure105, and the cycle starts over. The difference in pressure between EPAP105 and IPAP 115 is designated as ΔP in FIG. 1. This pressure changeoccurs instantaneously in the FIG. 1A theoretical model. Thus, FIG. 1Ashows pressure curve 100 as a square wave.

Patient comfort may not be optimized if the ideal pressure curve isapplied to the patient. More specifically, rather than an instantaneoustransition from EPAP to IPAP, patient comfort may be optimized if aslightly more gradual transition is made from EPAP to IPAP and viseversa. FIG. 1B shows such a ramping of a pressure curve 102 from EPAP105 to IPAP 115. This ramping effect is measured by the time it takesthe system pressure to increase from EPAP 105 to IPAP 115 and isreferred to as the “rise time” of the bi-level pressure support system.Similarly, rather than an instantaneous transition from IPAP to EPAP,FIG. 1B shows ramping of the system pressure from IPAP 115 to EPAP 105.This ramping effect is measured by the time it takes the system pressureto decrease from EPAP 105 to IPAP 115 and is referred to as the “falltime” of the bi-level pressure support system.

Accordingly, the rise time of a bi-level pressure support system isgenerally a measure of the time for the system pressure to change fromthe expiratory pressure to the inspiratory pressure, and fall time is ameasure of the time for the system pressure to change from theinspiratory pressure to the expiratory pressure. However, rather thanmeasure rise time from the peak-to-peak between EPAP and IPAP, the risetime can also be defined as the time it takes for the system pressure tochange from a percentage, such as 10%, of its initial pressure value toa percentage, such as 90%, of its final pressure value. With respect toa system pressure change (ΔP=IPAP−EPAP) from a lower EPAP to a higherIPAP value, the 10% initial pressure is defined as:

 EPAP+0.10×(IPAP−EPAP),

and the final 90% pressure is defined as:

EPAP+0.90×(IPAP−EPAP).

This 10% to 90% pressure change is shown in FIGS. 1B; 1C, and 1D asΔRTP.

Likewise, the fall time can also be defined as the time it takes for thesystem pressure to go from 90% of its initial pressure to 10% of itsfinal pressure when a fall from the higher IPAP value to the lower EPAPvalue is calculated. Thus, one may calculate separate rise and falltimes for each specific transition (low-to-high or high-to-low). Risetime is generally between 0.1 to 0.3 seconds for the pressure supportsystem of the present invention.

FIGS. 1B-1D each show the pressure differential from 10%-90% of finalIPAP value as ΔRTP and the corresponding rise time as ΔT. As shown inFIG. 1B, the cycle begins with the system pressure 102 at EPAP level105. When the pressure support system detects that the patient hasfinished expiration or has begun inspiration, or that some otherprompting event has occurred, the system pressure is ramped up towardIPAP level. The start of the rise time pressure ramp is indicated at 120in FIG. 1B. The ramping is shown as a straight line in FIG. 1B, but theramping may also be an exponential ramp or any other transitionalwaveform from one generally constant level to another. However, it is tobe understood that the IPAP and/or EPAP pressure need not necessarily beconstant. See, for example, U.S. Pat. Nos. 5,535,738 and 5,794,615, aswell as U.S. patent application Ser. No. 09/041,195, which teach varyingthe IPAP and/or EPEP pressure as a function of patient flow or apreestablished flow profile, the contents of each of which areincorporated herein by reference. Ultimately, the system pressurereaches its intended IPAP value at a point in pressure curve 102indicated at 125 and levels off at to IPAP 115. This smooth ramping ischaracterized by a rise time ΔT₁ and a pressure change ΔRPT.

It is generally believed that shorter rise times (ΔT) result in adecrease in comfort to the patient because of the “sharper” transient inpressure. However, shorter rise times are also believed to result in anincrease in system therapeutic effectiveness. FIG. 1C illustrates risetime ΔT₂ that begins at point 130 in pressure curve 104 and ends atpoint 135, so that rise time ΔT₂ is shorter than rise time ΔT₁, whichwill likely result in decreased patient comfort but increased therapyeffectiveness.

FIG. 1D illustrates rise time ΔT₃ that begins at point 140 in pressurecurve 106 and ends at point 145, so that rise time ΔT₃ is longer thanrise time ΔT₁, which will likely result in increased patient comfort,but decreased therapy effectiveness. It can thus be appreciated thatthere is a tradeoff between patient comfort (longer rise time) andeffective treatment (shorter rise time). As will be appreciated below,the present invention, in accordance with at least one presentlypreferred embodiment, seeks to maximize patient comfort whilemaintaining sufficiently effective respiratory treatment byautomatically selecting an optimal rise time to suit the needs of thepatient. The present invention also contemplates automatically adjustingthe fall time, such as to correspond to a mirror image of the pressurecurve during the rise time interval. Of course, the fall time can remainunchanged despite variations in the rise time.

Generally, in at least one embodiment of the present invention, the risetime of a ventilator or other pressure support system is automaticallyadjusted based on physiological conditions of the patient detected byflow, pressure, and other sensors. The rise time may preferably beginwith a maximal value, thus lending maximal user comfort, and decreasedas necessary, such as in accordance with the detection of apenic events,such as an apnea, hyponea, upper airway resistance, or snoring. Theevent detection can employ existing techniques, for example, based onflow limitations, upper airway noise, or both. If an apenic event isdetected, the rise time can be shortened or decremented. During thecontinued detection of apenic events or other detected problems, therise time may be shortened once per minute (or other time intervalaccording to a preset level) until it has been lowered to a minimum risetime. When five minutes (or some other preset amount of time) elapseswith no detection of an apenic event, the rise time is lengthened by apreset incremental amount. During the continued absence of apenicevents, this rise time lengthening continues until the rise time reachesa maximum allowed rise time. Preferably, the rise time minimum andincremental increase and decrease amounts are determined empirically viaclinical evaluation, but the maximum rise time is preferably calculateddynamically based on the inspiratory time of breathing.

The present invention contemplates that a rise time adjustment algorithmis processed continuously by the pressure support system duringoperation, resulting in continuous adjustment to the rise time tomaximize patient comfort while minimizing operator intervention. Therise time will vary as the algorithm determines the maximum possiblerise time for patient comfort while maintaining sufficient therapy.Because the rise time is automatically adjusted, the rise time necessaryfor sufficient therapy may be optimized from patient to patient, fromnight to night, and even from minute to minute on a particular patientand does not require any intervention of the patient or caregiver.

The present invention contemplates that the algorithm is implemented ina microprocessor-based bilevel pressure support system. Such a systempreferably has one or more sensors to produce a pressure signal, flowsignal, pressure and flow signals, or some other sensor signal,available to the microprocessor for triggering and cycling. The pressuresupport system also preferably includes control hardware to allow themicroprocessor to vary the rise time as determined by the implementationof the algorithm. For example, suitable pressure support devices thatcan be used to implement the automatic rise time control of the presentinvention are the BiPAP® Duets® Bi-level System or BiPAP® Duet® LXBi-level System, both of which are manufactured by Respironics, Inc., ofMurrysville, Pa. Such pressure support devices have a microprocessor,memory, pressure and flow sensors, and can be programmed in C or someother computer language.

FIG. 2 shows a general block diagram of a bi-level pressure systemsupport system 190 with automatic rise time adjustment according to onepreferred embodiment of the present invention. Bi-level pressure supportsystem 190, as shown, includes a pressure generating system 200, sensors204, represented by a pressure 205 and a flow 210 sensor, and acontroller 215 for controlling the operation of the ventilator and forcarrying out the automatic rise time adjustment. Bi-level pressuresupport system 190 also includes an input/output device 230, forexample, for inputting operating variables into the system and forreviewing its operation. A memory 240 stores such variables, as well asinformation gathered or determined by controller 215, such as-theoccurrence of apenic events. Bi-level pressure support system 190further includes a first timer 250 and a second timer 260, the operationof which are discussed in detail below.

In FIG. 2, the flow of pressurized air or other gas imparted to the useror patient is represented by an airflow arrow 220. In actual practice,this airflow 220 may be communicated to the user's respiratory system byway of a patient circuit, such as flexible plastic tubing, attached to apatient interface device, such as a nasal mask, nasal/oral mask, fullface mask, total face mask, hood, nasal cannula, tracheal tube, orendotracheal tube, for communicating a flow of gas in the patientcircuit with an airway of the user.

Pressure generating system 200 generates a flow of breathing gas that isprovided to the user. In a preferred embodiment of the presentinvention, pressure generating system 200 includes a means forgenerating a flow of breathing gas, such as blower, impeller, or fanrotated by a motor, a piston, or bellows to generate a flow of breathinggas. Because the pressure in a bi-level pressure support system changesdepending on whether the system is generating an expiratory or aninspiratory positive airway pressure, the pressure generating systempreferably includes a means for controlling the pressure of the flow ofbreathing gas communicated to the patient circuit, and, hence, to theairway of the patient. Typically, a constant pressure blower, i.e., ablower driven by a motor to output a constant pressure, is used togenerate a flow of breathing gas, and a pressure/flow control valve,which is typically although not necessarily, downstream of the constantpressure blower, controls the pressure of the flow of breathing gasprovided to the patient circuit, and, hence, provided to the airway ofthe patient. The blower itself can also provide different air pressures,for example by varying the operating speed of the blower. Blower speedand a pressure control valve can be used independently or in unison tocontrol the pressure of the flow of breathing gas delivered to theairway of the patient.

Sensors 204, such as flow sensor 210 and pressure sensor 205, preferablymonitor the conditions of the user's respiratory system to detect whenan apenic event occurs by monitoring the flow and pressure of breathinggas in the patient circuit. These sensors can be of numerous types, butflow and pressure sensors are common examples sensors well know foraccomplishing this functions. The output of these sensors is typicallyused to trigger and cycle the pressure support system or to perform afunction when an apenic event or other disturbance is detected, such asincrease the IPAP pressure. It is to be understood that the presentinvention contemplates using other patient monitoring devices as sensors204, and such sensors need not be necessary associated with the pressuregenerating system or patient circuit. For example, it is known to use aplethysmography belt, EMG sensors, motion detectors, temperature sensorsand other devices to detect patient respiration. It is also know todetect respiration based on the energy provided to the motor in thepressure support system. Thus, sensors 204 encompass any means fordetecting a physiological condition of the patient.

Generally, the above sensors, pressure/flow generating and deliveryarrangement, and control circuit work together as a positive airwaypressure support system. The sensors preferably detect variousconditions of the system and the patient's breathing pattern to provideinformation to the control system. The control system, which ispreferably microprocessor-based, uses this acquired information tomonitor the patient's breathing, for example, to determine whether anyapenic events or other conditions occur, and/or to control how the risetime and other aspects of the pressure delivered to the patient shouldbe altered, updated, or controlled. For instance, the control system maydetermine a new rise time based on the detection of an apenic event.This new rise time is communicated to the pressure delivery system toeither change the blower pressure directly or change the pressure valvesetting to impart a different air or gas pressure to the patient.

FIG. 3 shows an exemplary algorithm that controller 215 uses to adjustthe rise time of the bi-level pressure support system in accordance withone presently preferred embodiment of the invention. This algorithm isrepresented by a flow chart where the square chart entries represent asystem function and the diamond-shaped entries represent a decision thatthe control circuit makes.

The control system that operates according to the algorithm of FIG. 3may use several different variable values and system settings. First,there is an actual, or computed, system rise time. This value may becommunicated to the pressure delivery system and represents the risetime ΔT from FIGS. 1B-1D. This rise time exists between the range of twovariables: the maximum rise time and the minimum rise time. The minimumrise time is preset by either the patient's doctor or a knowledgeablepatient or other person and represents the shortest rise time that thebi-level pressure support system will allow. This limit is placed on thesystem for patient safety or comfort. On the other hand, the maximumrise time may either be preset, like the minimum rise time, or may bedetermined by the control system. Up to a point, the higher this maximumrise time, the greater the comfort the patient should enjoy while usingthe bi-level pressure support system. This maximum rise time value mayalso be limited.

Also, the bi-level pressure support system may have either a preset ordynamically controlled value for incrementing and/or decrementing thesystem rise time. When the controller determines that the rise time ofthe system pressure must be updated, the increment and decrement valuesrepresent a length of time by which the calculated rise time islengthened or shortened. These values may be set individually because,for example, the rise time decrement amount, which aids in therapy, maybe higher than the rise time increment amount, which is based on patientcomfort.

There may also be two rise time adjustment control variables based on adoctor or patient defined setting. The first variable, called forexample, “ApenicCheck,”determines the time period over which the systemwill check for apenic events and how many apenic events within that timeframe are sufficient to cause the system to alter the rise time. Thetime component of the ApenicCheck variable is preferably compared to arunning timer during system operation to determine when the window overwhich the control circuit examines the sensor data for apenic eventsopens and closes. This value may be preset and constant during systemoperation, or the value may be dynamically changed, for example, duringand soon after apenic events are sensed. The exemplary value of thisvariable in FIG. 3 is one minute. The number of apenic events componentof the ApenicCheck variable, which indicates how many apenic events canoccur within that time frame before the system will increment the risetime, may also be preset and constant during system operation, or thevalue may be dynamically changed, for example, during and soon afterapenic events are sensed. As described in greater detail below, in FIG.3, the presence of one apenic event within the one minute window may besufficient to cause an increase in the rise time.

The second variable, called, for example, “RiseTimeWait,” determines howlong the system will wait before lengthening the system rise time, and,therefore, make the patient more comfortable. This value will often beabove zero to allow the patient's respiratory system sufficient time toget used to the current rise time setting before a new rise time iscalculated. Again, this variable may be preset or dynamically changed.The example value of this variable in the algorithm of FIG. 3 is fiveminutes.

The bi-level pressure support system of the present invention operatesin accordance with the algorithm of FIG. 3 to control the rise time bystarting at step 310. In the step 320, each of two timers, Timer 1 andTimer 2, are set to an initial value of zero. These timers both begincounting the elapsed seconds up from zero after they are reset. Thefirst timer, Timer 1, is compared to the time component of theApenicCheck variable to determine the window over which the system needsto check for a recent apenic event. The second counter, Timer 2, iscompared to the RiseTimeWait variable to determine if the specifiedamount of time has elapsed since the last apenic event or the last risetime lengthening.

After resetting and starting the timers in step 320, the algorithm, instep 324, compares the first timer to the stored value of the timecomponent of the ApenicCheck variable to determine the time periodduring which the system monitors for apenic events. In FIG. 3, thisstored value is shown as one minute, but this value can be set to anydesired level. The time component of the ApenicCheck variable should beset to a value that allows the system to adjust the rise time toresponse relatively rapidly to changes in the patient's condition, whilenot overreacting to such changes. The time component of the ApenicCheckvariable should not be made too long or the system will not effectivelyresponse to the apenic events. On the other hand, time component of theApenicCheck variable should not be made too short or the system maychange the rise time too frequently, which may in turn wake or disturbthe patient.

If less than one minute has elapsed since the first timer has beenreset, the algorithm follows path 326 to decision block 350. If, on theother hand, one minute or more has elapsed on the first timer since thetimer has been reset, the algorithm follows path 328 to function block330.

In step 330, the first timer, Timer 1, is reset so that another oneminute interval can be counted off during which the system again checksfor an apenic event. The algorithm proceeds to step 332 where the inputfrom the flow, pressure, or other sensors is analyzed to determine ifone or more apenic events have been detected during the one minuteinterval. As described above, these sensors will preferably trigger aresponse from the control system when certain conditions, such as apenicevents, are detected. The number of such events in the set time frameare stored and compared to a threshold value in step 332. In theillustrated embodiment, if one apenic event occurs, the system proceedsto step 338. It is to be understood that the threshold number of apenicevents can be a value other than one, and this threshold number ofevents can be adjusted manually or dynamically during the operation ofthe system.

In FIG. 3, if no apenic event, or if the number of apenic events is lessthan the threshold requirement set in step 332 occur in the time frameset in step 324, the program follows path 334 back to block 324. In thiscase, no apenic events have occurred, the patient is resting quietly,and the rise time does not need to be decreased to increase the flow ofair to the patient. This is the normal flow of the rise time controlmethod or program.

If a check of the pressure, flow, or other sensors in block 332 showsthat an apenic event has occurred, path 336 is followed to functionblock 338 where the second timer, Timer 2, is reset to zero seconds.Timer 2 keeps track of the time since either an apenic event or a risetime increment has occurred. In step 338, Timer 2 is reset due to theoccurrence of the apenic event. After resetting the second timer, thesystem responds to the detected event by attempting to shorten thesystem pressure rise time (thereby increasing therapeutic effect). InFIG. 3, the algorithm proceeds to decision block 340, where the actualrise time is compared to its minimum allowed value set by the doctor orthe patient. If the rise time is at its minimum, then the rise timeshould remain at this minimum value despite the detection of a recentapenic event. Thus, path 344 is followed to return to decision block324. Although theoretically, the rise time should be shortened toincrease therapeutic effect, the automatic rise time adjustment programwill not decrease the rise time below the minimum safety values set bythe patient's doctor or by knowledgeable patients themselves.

If the rise time is not at its minimum allowed value, then the recentapenic event detected in block 332 causes the algorithm to proceed alongpath 342 to function block 346. At block 346, the rise time is decreasedby the preset decrement amount. As stated before, this amount can beselected by the user or the patient's doctor while initializing themachine, or the value can be dynamically set by the system. The risetime is decreased because the pressure and flow sensors detected anapenic event indicating that the patient is having trouble breathing atthe current rise time level. Hence, the shorter rise time will cause thebi-level pressure support system to change from one pressure level toanother in a shorter amount of time. Although less comfortable for thepatient, this decremental change may eliminate a more severe breathingepisode for the patient. This automatic adjustment may correct theapenic abnormality without waking or disturbing the patient. This makesthe overall system more responsive and comfortable.

After either adjusting the rise time value via path 348 or not adjustingthe rise time via path 344, the algorithm will return to decision block324. Here, Timer 1 is again compared to the time component of theApenicCheck variable. If Timer 1 is less than one minute, then decisionblock 324 will return a negative value, and the algorithm will proceedalong path 326 to the Timer 2 decision block 350. In block 350, theelapsed time of the second timer is compared to the RiseTimeWait value,which is shown as being five minutes in block 350. This block 350 willreturn a negative value if an apenic event or rise time increment hasoccurred within the last five minutes. If so, the algorithm will proceedalong path 352 back to the first timer decision block 324. If the secondtimer decision block 350 returns a negative value, then program path 354is followed to function block 356 where Timer 2 is reset to zeroseconds. This timer is reset because an apenic event has not occurredwithin five minutes and the rise time has not been shortened within fiveminutes.

At this stage, the control circuit has effectively determined that thepatient's breathing has been normal for at least five minutes, and theprogram seeks to increase patient comfort by increasing rise time. Thealgorithm proceeds to block 358 where the system rise time is comparedto the maximum rise time entered by the patient or her doctor. If therise time is at its maximum, then the patient is as comfortable as thepreset limits allow, and the automatic rise time adjuster will notincrease the rise time beyond this preset value. Even though a longerrise time theoretically may result in the patient being morecomfortable, the preset limits should not be traversed in the interestof patient safety. Hence, path 362 will be taken to step 363 where Timer1 is also reset to reestablish the window over which the system willcheck for apenic events and then the system will return back to thefirst timer comparison block 324, and the circuit will start over.

If the decision block 358 determines that the rise time is not at itspreset maximum value, then the algorithm proceeds along path 360 tofunction block 364. Here, the system rise time is incremented by thepreset amount to make the user more comfortable. In addition, Timer 1 isreset to reestablish the window over which the system will check forapenic events. The algorithm then follows path 366 and the cycle startsover in block 324 where Timer 1 is again compared to the time componentof the ApenicCheck variable.

The above algorithm has been provided for illustrative purposes only andshould not be construed to limit the present invention to any particularcombination of algorithm steps, variables, or algorithm procession. Theabove circuit may contain elements or steps that can be replaced byother steps or steps that may be ignored altogether. There are also manyadditional steps, variables, and elements that may be used in additionto the above system, and that are fully encompassed by the disclosureherein. For example, rather than look for x number of apenic events in ay time period, as discussed above with respect to steps 324 and 332, thesystem could look for the frequency at which such events are occurring.If they apenic events are occurring less than one minute apart, forexample, the rise time can be shortened.

It should be noted that the above control algorithm for the respiratorcontrol arrangement could be easily implemented on amicroprocessor-based system. In such a system, the microprocessor mayinclude a memory to store preset maximum and minimum system pressures,pressure increment/decrement values, or other values which arecalculated during ventilator operation. The microprocessor-based systemmay also include an arithmetic logic unit for calculating the updatedpressure rise time based on the values inputted to the microprocessor orstored in the memory. The function and structure of thismicroprocessor-based control arrangement may be complex, with additionalfeatures and components, and all such embodiments are contemplatedwithin the scope of this invention and disclosure.

Alternatively, the automatic rise time system could be hard-wired or mayexist as part of a non-microprocessor-based system. Simple electroniccomponents such as resistors, capacitors and inductors can be combinedin presently known ways to create timing circuits, and circuits thatrespond to input information. The above example was illustrated only byway of example to show one possible embodiment of the present invention.

Although the invention has been described in terms of particularembodiments in an application, one of ordinary skill in the art, inlight of the teachings herein, can generate additional embodiments andmodifications without departing from the spirit of, or exceeding thescope of, the claimed invention. Accordingly, it is understood that thedrawings and the descriptions herein are proffered by way of exampleonly to facilitate comprehension of the invention and should not beconstrued to limit the scope thereof.

What is claimed is:
 1. A pressure support system comprising: a pressuregenerating system adapted to produce a flow of breathing gas at aninspiratory positive airway pressure (IPAP) level and an expiratorypositive airway pressure (EPAP) level that is less than the IPAP level;a conduit operatively coupled to the pressure generating system todeliver the flow of breathing gas to an airway of a patient; a sensoradapted to detect a physiological condition associated with such apatient receiving the flow of breathing gas; and a processor receivingan output of the sensor and providing a control signal to the pressuregenerating system, wherein the processor is programmed to control thepressure generating system so as to: a) deliver the flow of breathinggas to such a patient at the IPAP level during at least a portion of aninspiratory phase of such a patient, and to deliver the flow ofbreathing gas to such a patient at the EPAP level during at least aportion of an expiratory phase of such a patient, and b) automaticallyadjust a slope of a transition of pressure from the EPAP level to theIPAP level based on the output of the sensor.
 2. The pressure supportsystem of claim 1, further comprising: an input/output deviceoperatively coupled to the processor; and at least one timer operativelyconnected to the processor, wherein the processor determines whether toadjust the slope of the transition of pressure from the EPAP level tothe IPAP level based on an output of the timer.
 3. The pressure supportsystem of claim 2, wherein the input/output device is used to set atleast one of: (1) a time interval during which the processor checks anoutput signal from the sensor to determine whether to adjust the slope,(2) a number of apenic events that can occur in the time interval beforethe processor will adjust the slope, and (3) an amount by which the isincremented or decremented by the processor.
 4. The pressure supportsystem of claim 1, wherein the processor dynamically determines at leastone of (1) a time interval during which the processor checks an outputsignal from the sensor to determine whether to adjust the slope, (2) anumber of apenic events that can occur in the time interval before theprocessor will adjust the slope, and (3) an amount by which the slope isincremented or decremented by the processor.
 5. The pressure supportsystem of claim 1, wherein the sensor is a flow sensor associated withthe conduit, a pressure sensor associated with the conduit, or both. 6.The pressure support system of claim 1, wherein the processor controlsthe pressure generating system so as to adjust a slope of a transitionof pressure from the IPAP level to the EPAP level.
 7. A pressure supportsystem comprising: pressure generating means for producing a flow ofbreathing gas at an inspiratory positive airway pressure (IPAP) leveland an expiratory positive airway pressure (EPAP) level that is lessthan the IPAP level; sensing means for detecting a physiologicalcondition associated with a patient receiving the flow of breathing gas;and controlling means, operatively connected to the sensing means andthe pressure generating means, for: a) determining a slope of atransition of pressure from the EPAP level to the IPAP level based on anoutput of the sensing means, b) causing the pressure generating means todeliver the flow of breathing gas to such a patient at the IPAP levelduring at least a portion of an inspiratory phase of such a patient, andto deliver the flow of breathing gas to such a patient at the EPAP levelduring at least a portion of an expiratory phase of such a patient, andc) controlling a rate of change from the EPEP level to the IPAP levelover the plurality of respiratory cycles based on the slope.
 8. Thepressure support system of claim 7, wherein the controlling meansincludes a micro processor.
 9. The pressure support system of claim 7,wherein the controlling means includes a memory for storing a value ofpressure support system variables.
 10. The pressure support system ofclaim 7, further comprising: inputting means, operatively coupled to thecontrolling means, for providing information thereto; and timing means,operatively connected to the controlling means, wherein the controllingmeans determines whether to adjust the slope of the transition ofpressure from the EPAP level to the IPAP level based on an output of thetiming means.
 11. The pressure support system of claim 7, wherein thesensing means comprises a flow sensor, a pressure sensor associated withthe pressure generating means, or both.
 12. The pressure support systemof claim 7, wherein the controlling means dynamically determines atleast one of: (1) a time interval during which the controlling meanschecks an output signal from the sensing means to determine whether toadjust the slope, (2) a number of apenic events that can occur in thetime interval before the controlling means will adjust the slope, and(3) an amount by which the slope is incremented or decremented by thecontrolling means.
 13. The pressure support system of claim 7, whereinthe controlling means also controls the pressure generating means so asto adjust a slope of a transition of pressure from the IPAP level to theEPAP level.
 14. A method for automatically adjusting a slope of apressure change in a pressure support system, comprising: producing aflow of breathing gas at an inspiratory positive airway pressure (IPAP)level and an expiratory positive airway pressure (EPAP) level that isless than the inspiratory positive airway pressure; detecting aphysiological condition associated with a patient receiving the flow ofbreathing gas; determining a slope of a transition of pressure from theEPAP level to the IPAP level based on the physiological conditionassociated with such a patient; delivering the flow of breathing gas tosuch a patient at the IPAP level during at least a portion of aninspiratory phase of such a patient, and delivering the flow ofbreathing gas to such a patient at the EPAP level during at least aportion of an expiratory phase of such a patient; and controlling a rateof change from the EPAP level to the IPAP level based on the slope. 15.The method according to claim 14, further including setting at least oneof (1) a minimum allowed slope, (2) a maximum allowed slope, (3) anamount for each incremental change in slope, and (4) an amount for eachdecremental change in slope.
 16. The method according to claim 14,further including dynamically determining at least one of (1) a timeinterval during which the physiological condition is checked, (2) anumber of apenic events that can occur in the time interval before theslope is adjusted, and (3) an amount by which the slope is incrementedor decremented.
 17. The method according to claim 14, wherein the slopeis decreased responsive to detecting an apenic event of a patient. 18.The method according to claim 14, wherein the slope is increased after aperiod of time in which no apenic events are detected.